One per month throughout 2010. So you've missed two; both will be available in a compilation in 2011.

To avoid scratching the collector/completist itch - I'll be glad to send you old playsets when you post some actual play. There are four excellent sets in the game itself! Enjoy them, have fun with the Playset of the Month, make your own, whatever, share your experience in some public forum, and ping me with the URL.